1. Field of the Invention
The present invention is in the field of electrocardiography and more specifically relates to apparatus for encoding, recording, playing back, decoding and displaying cardiac data, including selected ECG signals and blood pressure data.
2. The Prior Art
The present invention is a further step forward in the continuing development of instruments for ambulatory (Holter) monitoring. The present invention is in the nature of an innovative combination of certain prior art techniques, combining them with new techniques to obtain a new and useful result. For this reason, a knowledge of the salient prior art techniques is essential for understanding the present invention and viewing it from the proper perspective.
The U.S. Pat. No. 4,006,737 issued Feb. 8, 1977 to Cherry and assigned to Del Mar Engineering Laboratories, there was described an electrocardiographic computer for playing back, analyzing and displaying ECG signals recorded on a magnetic tape. Two channels of ECG signals are recorded on separate tracks in the magnetic tape. The recorder is not disclosed.
The electrocardiographic computer described in U.S. Pat. No. 4,006,737 includes provision for playing back the tape at a high speed so that the data may be quickly scanned by a skilled operator to determine which portions are significant. The electrocardiographic computer further includes provision for replaying at real time speed the portions of the tape deemed to be significant. When the tape is scanned at high speed, the computer calculates and continuously plots two channels of trend data such as heart rate and ST level. An arrhythmia computer within the electrocardiographic computer detects and digitally displays the number of premature ventricular contractions and the number of supraventricular ectopic beats and actuates an event marker to place a mark on the chart produced when the arrhythmia occurrences exceed a preselected number of occurrences during a predetermined time interval. When the tape is replayed at real time speed, the two channels of ECG data ae plotted. A printer indicates the time of day on the chart, but this time of day data is based on the amount of tape that has been played, as sensed by a tachometer system within the electrocardiographic computer.
U.S. Pat. No. 4,073,011 issued Feb. 7, 1978 to Cherry and Anderson describes an improved electrocardiographic computer as well as a recorder. The recorder again employs two tracks for recording on the tape. One channel of ECG signals is recorded on one of the tracks, while on the other track is recorded the second channel to ECG signals interrupted intermittently by event marker signals initiated by the patient. The recorder includes a time display so that the patient can note the time at which he initiated the event mark.
The electrocardiographic computer described in U.S. Pat. No. 4,073,011, on which the tape is played back, differs from the computer described in U.S. Pat. No. 4,006,737 in a number of improvements which cumulatively provide for a much greater degree of automatic processing of the tape so that the monitoring of the tape may be accomplished without the necessity of a technician's visually observing the oscilloscope or listening to an audible representation of the ECG signals. For example, during playback, the computer may be set to provide a trend run so as to print out a trend analysis from the beginning to the end of the tape and then have the tape stop automatically. Thereafter, the computer can be set to automatically cycle to the beginning of the tape to again print out the trend analysis but with an automatic detection of various events. The detection of the various events is used to trigger the computer so that the tape is slowed down to real time to print out the portion of the ECG signals bearing on the event. Thus, the technician does not have to monitor the playback to manually slow the tape down to real time as was done in the prior computer, but instead, the computer itself senses the occurrence of an event during trend and slows the tape down to print out in real time the ECG signals and then speeds back up to the orginally-selected trend speed.
The chart produced by the computer of U.S. Pat. No. 4,073,011 is the same as that produced by the computer of U.S. Pat. No. 4,006,737 during high-speed scan, but during real time playback the event marker signal initiated by the patient is displayed on one of the EGC traces in the computer, as described in U.S. Pat. No. 4,073,011.
In application Ser. No. 796,893 filed May 16, 1977 for "Blood Pressure Monitoring System," by Squires et al., there is disclosed a blood pressure measuring apparatus suitable for long-term ambulatory monitoring of the blood pressure of the patient to whom the device is affixed. The blood pressure measuring device is used in conjunction with a recorder of the type described in U.S. Pat. No. 4,073,011, with certain modifications made to the recorder. One of the modifications establishes the priority of the signals for recording. The recorder is capable of recording on two tracks of the tape simultaneously, and one of the tracks is dedicated to a first channel of ECG signals. On the second track of the tape a second channel of ECG signals is recorded, but is interrupted intermittently by the patient-initiated event marker signal and by the intermittently-generated blood pressure data signal. The blood pressure data signal has first priority followed by the event marker signal, and the lowest priority is given to the second channel of ECG signals.
The tape produced by the recorder used in conjunction with the blood pressure measuring apparatus is intended to be played back on the electrocardiographic computer described in U.S. Pat. No. 4,073,011 with slight modifications. That computer, as modified, permits a mark representing the blood pressure measurement to be placed on the heart rate trend chart during playback at .times.60 and .times.120 speeds, as shown in FIG. 15 of the application Ser. No. 796,893.
In the recorder described in U.S. Pat. No. 4,073,011, and used in the blood pressure apparatus, the event marker signal consists of a burst of eight cycles at a frequency of 8 Hz. When used in conjunction with the blood pressure measuring apparatus of Ser. No. 796,893, the recorder is modified to include a blood pressure encoder which encodes the systolic and diastolic blood pressure readings in a form particularly well suited for recording and subsequent decoding. The systolic reading includes eight binary bits and the diastolic reading includes another eight binary bits, so that the entire signal consists of sixteen bits which are produced at the rate of 16 Hz. The sixteen-bit blood pressure signal is preceded and followed by a one-eighth-second blanking preamble and postamble.
When used to play back tapes made in conjunction with the blood pressure measuring apparatus, the computer described in U.S. Pat. No. 4,073,011 is modified to include a decoder for decoding the blood pressure signal, which can be identified in that it consists of sixteen pulses at a 16-Hz rate.
The portable tape recorder disclosed in U.S. Pat. No. 4,073,011 and modified as described in application Ser. No. 796,893 to permit both an encoded blood pressure signal and an event marker signal to be placed on one of the tracks along with the ECG signal, is further modified and improved in the present invention, as will be described below. Likewise, the electrocardiographic computer described in U.S. Pat. No. 4,073,011 and modified as described in application Ser. No. 796,893 to provide for decoding of the blood pressure signal is further modified and improved in the present invention, as will also be described below. For these reasons, the descriptions given in U.S. Pat. No. 4,073,011 and application Ser. No. 796,893 filed May 16, 1977 are deemed to be incorporated herein for purposes of disclosure and to avoid unnecessary repetition of the background material.
In the recorder and the electrocardiographic computer described in U.S. Pat. No. 4,073,011 and in application Ser. No. 796,893, a magnetic recording tape of a standard width is used, and two tracks of data are recorded on it. This type of tape and the two-track recording format has become relatively standard and use of the electrocardiographic computers and recorders has become widespread.
For this reason, it was deemed essential that the apparatus of the present invention must be compatible with the equipment already in widespread use. Thus, it was essential that the apparatus of the present invention employ a magnetic tape of the same standard width and having two data tracks recorded on it. It was also deemed essential that the recorder of the present invention be usable to record the data in a form which can be analyzed with the existing electrocardiographic computers so far as possible, and that the electrocardiographic computer when modified in accordance with the present invention must still be able to play back and analyze tapes made on existing recorders.
In the electrocardiographic computers described in U.S. Pat. Nos. 4,006,737 and 4,073,011, the electrocardiographic computer kept track of the time of day at which the signals occurred through the use of a tachometer system driven by the tape as it is played back. In that system, the tape engages a capstan which produces rotation of an optical encoder which produces a bi-phase electrical pulse each time the tape has travelled an additional one-sixteenth inch. These tachometer pulses are used to increment a counter into which it was necessary for the operator to insert time of day.
It has been found from experience that the rubber portion of the capstan which engages the magnetic tape has a tendency to age and to wear with extended use. This, combined with friction in the tape path results in tape speed errors which are cumulative, and during a twenty-four-hour recording session, a one-percent change in tape speed will produce an accumulated error of fourteen minutes.
Thus, the present invention arose from the need to find a better way of determining the time at which the recorded signals occur, but as described above, it was recognized that whatever solution was implemented would have to be compatible with existing equipment.